Optically variable devices embedded in security documents are used to provide a high level of security whilst also providing an aesthetically pleasing effect.
Printed matter always has the problem of being copied or simulated by photocopying or scanning devices as well as simple printing techniques widely available in the commercial world. Therefore, devices that change colour or shape under various lighting conditions and or geometry make the task of counterfeiting or simulating the document much more difficult.
The introduction of the polymer security substrate has offered the perfect medium to produce secure devices in a cost effective and secure manner. As most high level security documents are already printed via the intaglio process, a well known method of printing which uses elevated temperatures and high pressures, 70°-90° C. at 25-30 Mpa, the machines and special inks for this process are only sold to bona fide security printers which offers a degree of inherent security.
In International Patent Application PCT/AU98/00046, a printed security document or device is described as including a reflective or brightly coloured base layer and a raised printed image applied to that layer by a printing process, at least part of the raised printed image having a height of at least 5 μm, the image being enhanced by the reflective or brightly coloured layer when viewed at different angles under different lighting conditions. Subsequent research on the effect created by this arrangement has revealed that it is important for best results for the base layer to be highly reflective and for the raised printed image to be printed in an ink having predetermined chroma and lightness.
It has now been determined that different effects can be achieved, while maintaining the same or better security, by changing the nature of the ink for producing the raised printed image.